Color in documents is the result of a combination of a limited set of colors over a small area, in densities selected to integrate to a desired color response. This is accomplished in many printing devices by reproducing separations of the image, where each separation provides varying density of a single primary color. When combined together with other separations, the result is a full color image.
In the digital reproduction of documents, a separation is conveniently represented as a monochromatic bitmap, which may be described as an electronic image with discrete signals (hereinafter, pixels) defined by position and density. In such a system, density is described as one level in a number of possible states or levels. When more than two levels of density are used in the description of the image, the levels are often termed "gray", indicating that they vary between a maximum and minimum, and without reference to their actual color. Most printing systems have the ability to reproduce an image with a small number of levels, most commonly two, although other numbers are possible. Common input devices including document scanners, digital cameras and the computer imagery generators, however, are capable of describing an image with a substantially larger number of gray levels, with 256 levels a commonly selected number, although larger and smaller levels are possible. It is required that an image initially described at a large set of levels also be describable at a smaller set of levels, in a manner which captures the intent of the user. In digital reproduction of color documents this means that each of the color separations is reduced from the input number of levels to a smaller output number of levels. The multiple color separations are combined together at printing to yield the final color print. Commonly, color documents are formed using cyan, magenta and yellow colorants or cyan, magenta, yellow and black colorants. A larger number or alternative colorants may also be used.
Printers typically provide a limited number of output possibilities, and are commonly binary, i.e., they produce either a spot or no spot at a given location (although multilevel printers beyond binary are known). Thus, given an image or a separation in a color image having perhaps 256 possible density levels, a set of binary printer signals must be produced representing the contone effect. In such arrangements, over a given area in the separation having a number of contone pixels therein, each pixel value in an array of contone pixels within the area is compared to one of a set of preselected thresholds as taught, for example, in U.S. Pat. No. 4,149,194 to Holladay. The effect of such an arrangement is that, for an area where the image is a contone, some of the thresholds will be exceeded, i.e. the image value at that specific location is larger than the value of the threshold for that same location, while others are not. In the binary case, the pixels or cell elements for which the thresholds are exceeded might be printed as black or some color, while the remaining elements are allowed to remain white or uncolored, dependent on the actual physical quantity described by the data. The described halftoning or dithering method produces an output pattern that is periodic or quasiperiodic in the spatial coordinates.
U.S. Pat. No. 5,673,121 discloses an idealized stochastic screen is characterized by all of the predominant color dots (black or white) uniformly distributed. The present invention seeks to approach this optimization by iteratively selecting pairs of threshold levels in the screen matrix, and measuring the approach to the idealized stochastic screen. The threshold values are then swapped in position to determine whether the swap improves the measurement or not. If it does, the swap is maintained. The process is iterated until the desired result is obtained.
When halftone-based printers are used for engineering drawings, the most critical requirement for halftoning is the quality of reproducing fine lines, which can be in arbitrary angles, in any colors and often only in few-pixel wide. Since stochastic screens provide highest resolutions at all possible levels with all possible orientations, they are widely accepted as the best choice for halftoning vector inputs such as engineering drawings.
The design criteria for stochastic screens require all minority pixels, black or white, are separated as much as possible. For most input levels the outputs of stochastic screens have "blue-noise" spatial spectra which provide pleasant appearance. However, due to the raster-structure limitation, near the theoretical 50% input level the design criteria force the halftone output toward a checkerboard pattern, which possesses only two equal spatial frequencies at orthogonal directions. FIG. 1 shows a typical stochastic halftone output with a spatial constant input at the 50% level. Unfortunately, checkerboard patterns are often used in engineering drawings to represent different gray levels or colors. When the input is gray checkerboard pattern, i.e., at least one of the two levels is neither 0 nor 100%, the halftone output by a stochastic screen show noisy beating, or moire, between the input and the screen. For example, FIG. 2 shows the output of a 0/50% checkerboard input halftoned by the same stochastic screen used for FIG. 1.
The above references are herein incorporated by reference for their teachings.